Still need to know how to do it? Flip the seat over and look on the bottom. You'll see the two plastic parts that hold the seatbelt to the bottom. Pop them out (I use a butterknife to do this), and then feed the pieces through the bottom of the seat. Flip the seat back over, and feed the straps, and the plastic clip, through the slits in the back of the seat (near baby's shoulders). Flip the seat over again, and look at the back. When you pull the pieces through the slits, you can then re-insert them into a higher set of slits. Then just go backwards. Weave the straps back into the holes near where the baby's hips would go, and then re-clip the plastic parts on the bottom of the seat.
I hope I didn't over-complicate that. :LOL It was a giant pain to do it the first time, but it's pretty easy now that I've done it so many times. Oh, and in case you didn't know, you want the straps in a hole that is still lower than the baby's shoulders. So whichever set of slits is the highest one that STILL goes below the baby's shoulders, is the proper setting.